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MODULE 14

C++ INHERITANCE 1: CONCEPTS WITH CODE SAMPLES

My Training Period: yy hours.

The source code for this tutorial is available in C++ Inheritance source codes.

The C++ programming abilities that supposed to be acquired:

Able to understand the inheritance concept.
Able to understand and use base class (parent class).
Able to understand and use derived class (child class).
Able to understand the use of the preprocessor directive to avoid the multiple inclusion of the same file.
Able to understand the class hierarchy.

10.1 Introduction To Inheritance

Inheritance permits to re-use code from the already available classes, also gives you the flexibility to do modification if the old code doesn’t exactly fit the task of the new project.
It doesn’t make sense to start every new project with new classes from scratch since most of the code will certainly be repeated in several programs such as common tasks or routines.
You are less likely to make an error if you leave the original alone and only add new features to it. Another reason for using inheritance is if the project requires the use of several classes which are very similar but slightly different.
In this Module, we will concentrate on the mechanism of inheritance and how to build it into a program.
C++ allows you to inherit all or part of the members variables and methods of classes, modify some, and add new ones that not available in the base class.
Keep in mind that in the real programming it is rare for developers to develop their own classes because every development tool or compilers will have abundant of-the-shelf class library that have been tested and ready to be used. For example Microsoft Foundation Class (MFC), .NET class library, graphic library etc.

14.2 The Starting Point

Examine the file named vehicle.h for simple class which we will use to begin our study of inheritance. It consists of four simple methods which can be used to manipulate data pertaining to our vehicle.

1. // a class declaration part, vehicle.h, header file

2. // save and include this file in your next project

3. // do not compile or run

5. #ifndef VEHICLE_H //preprocessor directive

6. #define VEHICLE_H

8. // class declaration part – the interface

9. class Cvehicle

10. {

11. protected:

12. int wheels;

13. int weight;

14. public:

15. void initialize(int input_wheels, float input_weight);

16. int get_wheels(void);

17. float get_weight(void);

18. float wheel_load (void);

19. };

20. #endif

20 Lines of codes

Program 14.1: vehicle.h program, declaration of Cvehicleclass

We will eventually refer to this as a base class or parent class, but for the time being, we will simply use it like any other class to show that it is indeed just a normal class.
Note that we will explain the added keyword protected shortly. Figure 14.1 is a graphical representation of the Cvehicle class. Ignore lines 5, 6, and 11, they will be explained in detail later. This program cannot be compiled or executed because it is only a header file without the main() but makes sure there are no error such as typing errors.

----------------------------------------------------------------------------

C++ cvehicle class inheritance illustration

14.3 The Cvehicle Class Implementation – vehicle.cpp

Next, examine the file named vehicle.cpp, and you will find that it is the implementation of the vehicle class. Theinitialize() method assigns the values input as parameters to the wheels andweight variables.

1. // program vehicle.cpp, implementation part,

2. // compile without error, generating object file, do not run

3. #include "vehicle.h"

5. // class implementation part

6. // initialize to data input by user

7. void Cvehicle::initialize(int input_wheels, float input_weight)

8. {

9. wheels = input_wheels;

10. weight = input_weight;

11. }

12.

13. // get the number of wheels of this vehicle

14. int Cvehicle::get_wheels()

15. {

16. return wheels;

17. }

18.

19. // return the weight of this vehicle

20. float Cvehicle::get_weight()

21. {

22. return weight;

23. }

24.

25. // return the load on each wheel

26. float Cvehicle::wheel_load()

27. {

28. return (weight/wheels);

29. }

29 Lines of codes

Program 14.2: vehicle.cpp, implementation program of Cvehicle class

We have methods to return the number of wheels and theweight, and finally, we have one that does a calculation to return the load on each wheel. At this point, we are more interested in learning how to implement the interface to the classes.
We will use it as a base class later. Compile this program without error, generating object file, in preparation for the next program example, but you cannot execute it because there is no main() entry point.

14.4 Using The Vehicle Class – The main() program

The file namedtransprt.cpp uses the Cvehicle class in similar manner as we have illustrated in the previous Module. This should be an indication to you that the Cvehicle class is just a normal class as defined in C++.

1. // program transprt.cpp, the main program,

2. // compile and run

4. #include <iostream>

5. using namespace std;

6. #include "vehicle.h"

7. // a user define header file and put it in the same folder as this program

9. void main()

10. {

11. Cvehicle car, motorcycle, truck, sedan_car;

12. // 4 objects instantiated

13.

14. // data initialization

15. car.initialize(4,3000.0);

16. truck.initialize(20,30000.0);

17. motorcycle.initialize(2,900.0);

18. sedan_car.initialize(4,3000.0);

19.

20. // display the data

21. cout<<"The car has "<<car.get_wheels()<< " tires.\n";

22. cout<<"Truck has load "<<truck.wheel_load()<<" kg per tire.\n";

23. cout<<"Motorcycle weight is "<<motorcycle.get_weight()<<" kg.\n";

24. cout<<"Weight of sedan car is "<<sedan_car.get_weight()<<" kg, and has

25. "<<sedan_car.get_wheels()<<" tires.\n";

26.

27. // system("pause");

28. }

28 Lines: Output:

C++ cvehicle class inheritance main program

Program 14.3: transprt.cpp the main program

Inheritance uses an existing class and adds new functionalities to the new class, to accomplish another, possibly more complex task.
This program declares four objects of the Cvehicle class as shown in the code segment below:

Cvehicle car, motorcycle, truck, sedan_car;

// 4 objects instantiated

Initializes them as shown below:

// data initialization

car.initialize(4, 3000.0);

truck.initialize(20, 30000.0);

motorcycle.initialize(2,900.0);

sedan_car.initialize(4, 3000.0);

And prints out a few of the values. Compile and run this program.

14.5 A Derived Class

Examine the following file named car.h, for our first example of using a derived class. The Cvehicle class is inherited due to the ":public Cvehicle" code added to line 12 as shown below:

class Ccar : public Cvehicle

1. // another class declaration car.h

2. // save and include this file in your project

3. // do not compile or run.

5. #ifndef CAR_H // to avoid car.h re-inclusion/multi inclusion

6. #define CAR_H

8. #include "vehicle.h"

10. // a derived class declaration part

11. // Ccar class derived from Cvehicle class

12. class Ccar : public Cvehicle

13. {

14. int passenger_load;

15. public:

16. // this method will be used instead of the same

17. // method in Cvehicle class - overriding

18. void initialize(int input_wheels, float input_weight, int people = 4);

19. int passengers(void);

20. };

21.

22. #endif

22 Lines of codes

Program 14.4: car.h program, declaration of derived class Ccar

This derived class named Ccar is composed of all the information included in the base class Cvehicle, and all of its own additional information. Even though we did nothing to the class named Cvehicle, we made it available in this car.h program.
In fact, it can be used as a normal class and a base class in the same program.
A class that inherits another class is called a derived class or a child class.
Likewise the terminology for the inherited class is called a base class, butparent class and super class are sometimes used.
A base class is rather general class which can cover a wide range of objects attributes or behavior or properties, whereas a derived class is somewhat more specific but at the same time more useful.
For simple example consider your family. You and your siblings inherited many characteristics of your mother and/or father such as eye and hair colors. Your father and/or mother are base classes, whereas you and your siblings are derived classes.
In this case, the Cvehicle base class can be used to declare objects that represent plane, trucks, cars, bicycles, or any other vehicles you can think up.
The class namedCcar however can only be used to declare an object that is of type Ccar because we have limited kinds of data that can be intelligently used with it. The car class is therefore more restrictive and specific than theCvehicle class.
If we wish to get even more specific, we could define a derived class usingCcar as the base class, name itCsports_car, and include information such asred_line_limit for the tachometer orturbo_type_engine as new member variables.
Then, theCcar class would therefore be used as a derived class and a base class at the same time, so it should be clear that these names refer to how the class is used.

14.6 A Derived Class Declaration

A derived class is defined by including the header file of the base class as is done in line 8 as shown below:

#include "vehicle.h"

And then the name of the base class is given following the name of the derived class separated by a colon ( : ) as is illustrated in line 12 as shown below:

class Ccar : public Cvehicle

Ignore the keywordpublic immediately following the colon in this line. This defines public inheritance and we will discuss it later.
All objects declared as being of class Ccar therefore are composed of the two variables from the Cvehicle class because they inherit those variables, and the single member variable declared in the Ccar class namedpassenger_load, and the total member variable is listed below:

▪ int wheels;

▪ int weight;

▪ int passenger_load;

An object of this class also will have three of the four methods ofCvehicle as listed below:

▪ get_wheels( ) { }

▪ get_weight( ) { }

▪ wheel_load( ) { }

And the two new methods:

▪ initialize( ) { }

▪ passengers( ) { }

The method namedinitialize() which is part of theCvehicle class will not be available here because it is hidden by the local version of initialize() which is a part of the Ccar class. The local method will be used if the name is similar, allowing you to customize your new class. Figure 14.2 is a graphical representation of an object of this class.
Note once again that the implementation for the base class only needs to be supplied in its compiled form of the vehicle.cpp file. The source code for implementation can be hidden.
The header file for the base class, vehicle.h, must be available as a text file since the class definitions are required in order to use the class.

C++ Ccar class illustration

The following figure is an illustration of our simple class hierarchy. Here, car inherits some of the common vehicle’s properties.

a very simple C++ class hierarchy

tenouk C++ on object oriented: Inheritance principles

Further C++ inheritance related reading:

TheVisual C++ .NET.
Check the best selling C / C++ and object oriented books at Amazon.com.
See MFC library class hierarchy chart here.
The source code for this tutorial is available in C++ Inheritance source codes.
Find a lot of information about C++ history and evolution at Bjarne Stroustrup site(http://www.stroustrup.com/).

|< C++ Data Encapsulation 7 | Main | C++ Inheritance 1a >|Site Index |Download |

C++ Class Inheritance: Part 1 | Part 2 |

|< C++ Data Encapsulation 7 | Main | C++ Inheritance 1a >|Site Index |Download |

MODULE 14

C++ INHERITANCE 1: CONCEPTS WITH CODE SAMPLES

My Training Period: yy hours.

The C++ programming abilities that supposed to be acquired:

Able to understand the inheritance concept.

Able to understand and use base class (parent class).

Able to understand and use derived class (child class).

Able to understand the use of the preprocessor directive to avoid the multiple inclusion of the same file.

Able to understand the class hierarchy.

10.1 Introduction To Inheritance

14.2 The Starting Point

20 Lines of codes

14.3 The Cvehicle Class Implementation – vehicle.cpp

29 Lines of codes

14.4 Using The Vehicle Class – The main() program

28 Lines: Output:

14.5 A Derived Class

22 Lines of codes

14.6 A Derived Class Declaration

Further C++ inheritance related reading:

|< C++ Data Encapsulation 7 | Main | C++ Inheritance 1a >|Site Index |Download |

C++ Class Inheritance: Part 1 | Part 2 |